The present invention relates to the extraction of mineral values from mineral-containing ores. In a more specific aspect, the present invention relates to the extraction of uranium values from uranium-containing ores.
Numerous minerals are present in subsurface earth formations in very small quantities which make their recovery extremely difficult. However, in most instances, these minerals are also extremely valuable, thereby justifying efforts to recover the same. An example of one such mineral is uranium. However, numerous other valuable minerals, such as copper, nickel, molybdenum, rhenium, silver, selenium, vanadium, thorium, gold, rare earth metals, etc., are also present in small quantities in subsurface formations, alone and quite often associated with uranium. Consequently, the recovery of such minerals is fraught with essentially the same problems as the recovery of uranium and, in general, the same techniques for recovering uranium can also be utilized to recover such other mineral values, whether associated with uranium or occurring alone. Therefore, a discussion of the recovery of uranium will be appropriate for all such minerals.
Uranium occurs in a wide variety of subterranean strata such as granites and granitic deposits, pegmatites and pegmatite dikes and veins, and sedimentary strata such as sandstones, unconsolidated sands, limestones, etc. However, very few subterranean deposits have a high concentration of uranium. For example, most uranium-containing deposits contain from about 0.01 to 1 weight percent uranium, expressed as U.sub.3 O.sub.8 as is conventional practice in the art. Few ores contain more than about 1 percent uranium and deposits containing below about 0.1 percent uranium are considered so poor as to be currently uneconomical to recover unless other mineral values, such as vanadium, gold and the like, can be simultaneously recovered.
There are several known techniques for extracting uranium values from uranium-containing materials. One common technique is roasting of the ore, usually in the presence of a combustion supporting gas, such as air or oxygen, and recovering the uranium from the resultant ash. However, the present invention is directed to the extraction of uranium values by the utilization of aqueous leaching solutions. There are two common leaching techniques for recovering uranium values, which depend primarily upon the accessibility and size of the subterranean deposit. To the extent that the deposit containing the uranium is accessible by conventional mining means and is of sufficient size to economically justify conventional mining, the ore is mined, ground to increase the contact area between the uranium values in the ore and the leach solution, usually less than about 14 mesh but in some cases, such as limestones, to nominally less than 325 mesh, and contacted with an aqueous leach solution for a time sufficient to obtain maximum extraction of the uranium values. On the other hand, where the uranium-containing deposit is inaccessible or is too small to justify conventional mining, the aqueous leach solution is injected into the subsurface formation through at least one injection well penetrating the deposit, maintained in contact with the uranium-containing deposit for a time sufficient to extract the uranium values and the leach solution containing the uranium, usually referred to as a "pregnant" solution, is produced through at least one production well penetrating the deposit. The present invention is directed to the former, i.e., the leaching of ores.
The most common aqueous leach solutions are either aqueous acidic solutions, such as sulfuric acid solutions, or aqueous alkaline solutions, such as sodium carbonate and/or bicarbonate.
Aqueous acidic solutions are normally quite effective in the extraction of uranium values. However, aqueous acidic solutions generally cannot be utilized to extract uranium values from ore or in situ from deposits containing high concentrations of acid-consuming gangue, such as limestone. Aqueous alkaline leach solutions are applicable to all types of uranium-containing materials and are less expensive than acids.
The uranium values are conventionally recovered from acidic leach solutions by techniques well known in the mining art, such as direct precipitation, selective ion exchange, liquid extraction, etc. Similarly, pregnant alkaline leach solutions may be treated to recover the uranium values by contact with ion exchange resins, precipitation, as by adding sodium hydroxide to increase the pH of the solution to about 12, etc.
As described to this point, the extraction of uranium values is dependent to some extent upon the economics of mining versus in situ extraction and the relative costs of acidic leach solutions versus alkaline leach solutions. However, this is an oversimplification, to the extent that only uranium in its hexavalent state can be extracted in either acidic or alkaline leach solutions. While some uranium in its hexavalent state is present in ores and subterranean deposits, the vast majority of the uranium is present in its valence states lower than the hexavalent state. For example, uranium minerals are generally present in the form of uraninite, a natural oxide of uranium in a variety of forms such as UO.sub.2, UO.sub.3, UO.U.sub.2 O.sub.3 and mixed U.sub.3 O.sub.8 (UO.sub.2.UO.sub.3), the most prevalent variety of which is pitchblende containing about 55 to 75 percent of uranium as UO.sub.2 and up to about 30 percent uranium as UO.sub.3. Other forms in which uranium minerals are found include coffinite, carnotite, a hydrated vanadate of uranium and potassium having the formula K.sub.2 (UO.sub.2).sub.2 (VO.sub.4).sub.2.3H.sub.2 O, and uranites which are mineral phosphates of uranium with copper or calcium, for example, uranite lime having the general formula CaO.2UO.sub.3.P.sub.2 O.sub.5.8H.sub.2 O. Consequently, in order to extract uranium values from ores with aqueous acidic or aqueous alkaline leach solutions, it is necessary to oxidize the lower valence states of uranium to the soluble, hexavalent state.
Combinations of acids and oxidants which have been suggested by the prior art include nitric acid, hydrochloric acid or sulfuric acid, particularly sulfuric acid, in combination with air, oxygen, sodium chlorate, potassium permanganate, hydrogen peroxide and magnesium dioxide, as oxidants. Alkaline leachates and oxidants heretofore suggested include carbonates and/or bicarbonates of ammonium, sodium or potassium in combination with air, oxygen or hydrogen peroxide, as lixivants. However, sodium bicarbonate and/or carbonate have been used almost exclusively in actual practice.
Numerous problems obviously arise in the leaching of uranium values from uranium-containing ores. One of the most obvious is, of course, the large quantities of ores being handled and treated compared with the amount of uranium recovered. Such large quantities of ores make it costly to crush and grind the same to a size which can be effectively leached in a relatively short period of time. For example, as previously pointed out, leached ore should be reduced in size to less than about 14 mesh, but an even smaller size, in the neighborhood of 100 to 400 mesh, or smaller, would be ideal. The cost of the latter, however, becomes prohibitive. It is, therefore, desirable to reduce the degree of grinding necessary. In addition, it would be highly desirable to reduce the quantities of ores handled in any given step of the process.
The large quantities of ores being treated also increase the amounts of leachant or lixivants and oxidants required in order to recover a given amount of uranium and/or attain such recovery in a reasonable time. Thus, it is also highly desirable to reduce the amounts of leachant or lixivant and oxidant to a minimum for effective results.
While the leaching operation can be carried out at temperatures from atmospheric temperature up to about the boiling point of water, it is known that the higher the temperature, the more effective and more rapid the leaching. Consequently, the usual range of temperatures is between about 80.degree. and about 100.degree. C. While this temperature range appears modest for most chemical operations, in the leaching of uranium-containing ore, the temperature becomes a very significant problem. This is true since, at the high temperatures employed, the cost of materials of construction of the leaching tanks is a major factor. For example, it is necessary to use rubber lined stainless steel tanks and the manufacturers of such tanks will not assure reasonable lifetimes for the linings. Consequently, the utilization of less expensive equipment is desirable and even a small reduction in the temperature of the leaching operation can substantially reduce equipment costs and lengthen equipment life.
While it is relatively easy to recover 50% to 60% of the uranium content of an ore, at relatively low temperatures, with relatively low concentrations of leach solution and in relatively short periods of time, such recoveries are not acceptable in industrial operations. For an economic operation, recoveries in excess of about 85% of the original uranium are required and usually above 90%. This, again, contributes substantially to the cost of leach solutions. Also, as in any other operation of this type, it is relatively easy to approach the desired and economic recoveries, but it is most difficult to attain recovery of those last small increments which are necessary or desirable for an effective and economic operation.
As in any industrial operation, the time required or rate is a significant economic factor. Consequently, it would also be highly desirable to be able to increase the rate of recovery, even though no greater recovery is obtained. Here again, even a very small increase in rate makes a substantial difference in the overall operation.